PROJECT SUMMARY The niche is a critical regulator of long-term stem cell function. Our long-term goal is to understand the role the niche plays in regulation of muscle stem cell function. SCs reside in a quiescent state and undergo a series of transitions as they activate and proliferate. We have identified Wnt4 as a tissue resident inhibitory factor that represses activation and migration of quiescent satellite cells (QSCs) via Rho-FAK signaling. This is the first demonstration of a single paracrine acting niche factor coordinates the growth, migration and activation of QSCs. The overall goal of this proposed study is therefore to understand how Wnt4-Rho-FAK signaling axis represses activation and migration in QSCs and its implications for SC function in adult and aged mice. The first aim will combine temporal regulated and cell specific genetic strategies to modulate Wnt4 levels in muscle fibers to determine the role of Wnt4 (in muscle fibers) and Rho-FAK (in QSCs) on activation and migration in QSCs and the response to injury. The second aim will dissect the molecular mechanism that prevents QSC activation and migration via Wnt4. We will examine the role of Wnt4-Rho signaling in the repression of mTORC1 and Hippo signaling in QSCs. Understanding the role of the niche on SC function will lead to important insights for cell therapy, muscle degenerative diseases and aging.